A determination of the crankshaft angle is of central importance in terms of controlling internal combustion engines. Approaches known in the existing art utilize, in particular, incremental sensors on the crankshaft and/or camshaft. Sensor disks having increment marks encompassing teeth and tooth gaps, which in coaction with the signals from the crankshaft and camshaft enable a determination of the engine position, are usual.
Sensor systems of this kind allow a determination of the absolute position of the crankshaft by way of a non-uniform placement of the increment marks. A typical implementation is a sensor wheel having 60 minus 2 teeth, i.e. 58 teeth, and a two-tooth sensor-wheel gap.
A disadvantage of such a gap is the absence of increments for exact determination of the crankshaft angle within the gap. Within the gap, an extrapolation of the crankshaft angle is performed by the engine controller, although that extrapolation is error-prone because of the non-uniformity of the crankshaft angle speed. Modern working methods for internal combustion engines impose more stringent requirements in terms of accuracy, in particular for determining the location of injection operations, for both Otto-cycle and diesel engines.
It is possible to avoid the sensor-wheel gap by an asymmetrical pitch of the increment markings and thus an asymmetrical pitch of the teeth with respect to tooth gaps. Instead of a sensor-wheel gap, here the pitch between teeth and tooth gaps is modified over one or more tooth/tooth-gap pairs. To increase the accuracy with which this modification is detected, usually the teeth and tooth gaps together are already configured asymmetrically, and the asymmetry in the region replacing the previous sensor-wheel gap is, for example, simply turned around. For example, if the teeth extend over a crankshaft angle of 4°, and the tooth gaps over a crankshaft angle of 2°, then the sensor-wheel gap is replaced by a reversal of this ratio, i.e. by teeth of, for example, 2° and tooth gaps of, for example, 4°.
With evaluation of the trailing tooth edges, a signal is available for an engine controller every 6° of crankshaft angle, even in the former gap, and in addition a demonstrable position of the sensor wheel is detected by evaluating the ratio between the tooth time and gap time.
Additional evaluation of the ratio between tooth time and gap time is complicated by what is today the typical embodiment of the crankshaft angle sensor. As a rule, these sensors are so-called differential sensors, in which signal processing encompasses appropriate calculation of a difference between sensor elements that are spatially separated from one another. An important advantage as compared with a so-called single sensor having only one sensor element is greatly improved reproducibility of the sensor signal. Improved reproducibility means a decrease in statistical errors upon acquisition and sampling of the sensor wheel's increment markings. Any changes in the magnetic field due to external interference fields or changes in the air gap of the sensor wheel have an effect on the switching threshold of the sensor in the case of a single sensor, but cancel each other out in the case of a differential sensor. The differential sensor is therefore more robust in terms of installation positions and external magnetic fields. The differential sensor switches at the center of the tooth or tooth gap at the zero transition of the differential signal. An asymmetrically apportioned sensor wheel generates an asymmetrical output signal only once, upon the transition from one tooth-to-space pitch to a different tooth-to-space pitch. A ratio of tooth time to gap time that is not equal to one is therefore available to an evaluation function in the engine controller only at the beginning and the end of the modified tooth pitch replacing the previous sensor-wheel gap. In terms of sensing of the rest of the sensor wheel, the tooth-to-gap time is approximately the same despite the asymmetrical tooth pitch.
It is furthermore known additionally to evaluate the rotation direction of the crankshaft. One known approach to transferring this information to an engine control unit is to use a variable pulse length. Because the engine control unit employs only one of the edges, usually the leading edge, for incremental determination of the angle traveled by the crankshaft, the other edge can be used to code further data. The rotation direction is transferred in this case by way of a change in the pulse length.
The existing art is disadvantageous in that evaluation of a sensor-wheel gap, or of a modified pitch of the sensor wheel replacing the sensor-wheel gap, is possible only on the basis of detection of the leading edges that are transmitted to an engine control unit.